In general, a lithographic printing plate consists of an ink-receptive image area for receiving an ink in the printing process and a hydrophilic non-image area for receiving a fountain solution. Lithographic printing is a method utilizing the nature of water and oily ink repelling each other, where the ink-receptive image area and the hydrophilic non-image area of a lithographic printing plate are used as an ink-receiving area and a fountain solution-receiving area (a non-ink-receiving area), respectively, to make a difference in adherence of the ink on the surface of the lithographic printing plate and after depositing the ink only on the image area, the ink is transferred to a printing material such as paper, thereby performing the printing.
For producing such a lithographic printing plate, conventionally, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an ink-receptive photosensitive resin layer (image recording layer) has been used, and a lithographic printing plate has been obtained by exposing the PS plate through a mask such as lith film and then performing a development processing with an alkaline developer or the like to dissolve and remove the unnecessary image-recording layer corresponding to the non-image area while leaving the image-recording layer corresponding to the image area.
The recent progress in this field makes it possible to obtain the lithographic printing plate by a CTP (computer-to-plate) technology at present. That is, a lithographic printing plate precursor is directly scanned and exposed without the intervention of a lith film by using a laser or a laser diode and then developed, whereby a lithographic printing plate is obtained.
With the progress above, the issue related to the lithographic printing plate precursor is changed to improving image-forming characteristics, printing characteristics, physical characteristics and the like responding to the CTP technology. Also, as another issue related the lithographic printing plate precursor, the increasing concern for global environment draws prominent attention to an environmental problem of waste liquid associated with a wet treatment such as development processing.
To cope with the environmental problem above, it is demanded to simplify the development or plate making or eliminate the need for processing. As one of simple plate making methods, a method called “on-press development” is being done. More specifically, this is a method where the lithographic printing plate precursor after exposure is directly loaded on a printing machine without performing conventional development and the unnecessary portion of the image-recording layer is removed at an early stage of the normal printing process.
For such simplification of the plate making work, a system using a lithographic printing plate precursor capable of being handled in a bright room or under a yellow lamp and a light source is preferred in view of ease of working. On this account, as for the light source, a semiconductor laser emitting an infrared ray with a wavelength of 760 to 1,200 nm or a solid laser such as YAG laser is used. A UV laser can be also used.
As regards the on-press developable lithographic printing plate precursor, for example, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-recording layer (heat-sensitive layer) containing a microcapsule encapsulating a polymerizable compound is described in Patent documents 1 and 2. Also, a lithographic printing plate precursor comprising a support having provided thereon an image-recording layer (photosensitive layer) containing an infrared absorbing dye, a radical polymerization initiator and a polymerizable compound is described in Patent Document 3. Furthermore, an on-press developable lithographic printing plate precursor comprising a support having provided thereon an image-recording layer containing a polymerizable compound and a graft polymer having a polyethylene oxide chain in the side chain or a block polymer having a polyethylene oxide block is described in Patent Document 4.
In general, an operation (plate inspection) to inspect or discriminate the image on the printing plate for checking whether the intended image recording is made on the printing plate is performed as a pre-step before loading the printing plate on a printing machine. In the case of a normal lithographic printing plate precursor involving a development processing step, a stained image is generally obtained by development processing when the image-recording layer is colored, and therefore, it is easy to confirm the image before loading the printing plate on a printing machine.
However, in the case of a lithographic printing plate precursor of an on-press development type or a non-processing (non-development) type, which is not subjected to development processing, the printing plate at the stage of loading the printing plate on a printing machine has no image thereon, and plate inspection cannot be performed. In particular, whether a registry guide (register mark) as a registration landmark in multicolor printing can be distinguished or not is important for printing work. For this reason, the lithographic printing plate precursor of an on-press development type or a non-processing (non-development) type is required to provide means for confirming the image, that is, produce a so-called print-out image resulting from coloring or decoloring in the exposed region, at the stage of being exposed. Furthermore, from the standpoint of enhancing the workability, it is also required that the exposed region involved in coloring or decoloring is not changed even after the elapse of time and keeps the colored or decolored state.
A lithographic printing plate precursor using, as the print-out agent, a compound capable of generating an acid, a base or a radical by light or heat and a compound capable of interacting with the generated acid, base or radical to cause color change has been proposed (see, for example, Patent Document 5). Also, it has been proposed to utilize the change in color of a thermally decomposable compound as the print-out agent of a direct-drawing type lithographic printing plate precursor having a heat-sensitive layer (see, for example, Patent Document 6). Furthermore, it has been also proposed to use a thermally decomposable dye having a thermal decomposition temperature of 250° C. or less as the print-out agent (see, for example, Patent Document 7).
According to these techniques, coloring or decoloring occurs in the exposed area, and the suitability for plate inspection is enhanced to some extent, but this is still insufficient.
In Patent Document 8, it is stated that a print-out image having good visibility and being at a level allowing plate inspection is obtained by a system containing an infrared absorbing dye of a cyanine dye type having a 5-membered ring in the methine chain and a radical generator. However, this technique is insufficient in terms of maintaining good visibility even after the elapse of time.